Refractive Method

More detailed description of the refraction method is given on pp 353—54 and of reflection method on p 354... 

Some of the energy returns to the surface where it is detected by sensitive instruments. When these instruments are placed near the source, so as to record the energy traveling in an essentially vertical path, the technique is known as the reflection method. When the instruments are extended over large distances from the source, as compared to the depths of interest, the technique is known as the refraction method pp351—52)... 

The seismic prospectors first utilized the refraction method, and it had spectacular success in locating salt domes along the Gulf Coast during the period 1923 to 1928. Since 1936, the reflection method has dominated seismic prospecting (pp353—54)... 

Reflection and Refraction Methods. See under Seismic Prospecting in this Vol... 

The seismic refraction method Is based on several Important assumptions (1) layer acoustic velocities Increase with depth (2) sufficient velocity contrast exists between layers to discriminate between different strata of interests (3) and layers must be thick enough to permit detection. 

Several different types of sound energy (waves) are propagated through the earth. Seismic refraction methods are concerned primarily with the compresslonal wave energy, commonly called primary wave or P-wave. Primary waves move through... 

Measurements of e,.—Stability under pressure is the prime requirement for capacitance cells used to determine dielectric virial coefiScients. Cells of both parallel-plate and cylinder-within-a-cjirnder design, stable to a few parts in 10 and usable to over 200 atm, have been described by a number of authors. " Cells for use in the microwave region and cells for measuring refractivity > have also been described. Early measuremoits at radio frequencies relied on the heterodyne beat method, " but more recent work " has utilized the three-terminal transform ratio-aim technique developed by Cole and Gross. This second method eliminates difficulties due to stray capacitances and provides accuracies of better than 1 part in 10 . For an exceUent review of techniques at both ratfio and microwave frequencies see ch. 2 of ref. 53. For refractivity methods see refs. 45,46, and 54. 

Limitations to consider when evaluating the suitability of the seismic refraction method for a given site include the following ... 

By contrast, the refraction acoustical technique involves the recording of refracted sound waves from the seabed and subbottom. Compared with the reflection technique, the refraction technique requires stronger energy sources and takes more time. In addition, the source and detectors must be spaced further apart. However, the refraction method provides deeper subbottom penetration. It is not commonly used in offshore engineering work. A typical arrangement for a seismic refraction survey that shows the required energy source and receiver close to or in contact with the seafloor is shown in Figure 3.7. 

Refraction Method to determine the amount of ocular refractive error used to establish a prescription for corrective glasses or contact lenses. 

In the presence of acetone and its homologues, where the specific gravity-refraction method would be inapplicable owing to the third constituent, Hoff and Macoun have worked out a method for the determination of methyl and ethyl alcohols, based on the formation of non-volatile products when formaldehyde and acetone react in the presence of alkali. 

While for the seismic reflection method, the reflected signal of an acoustic wave is studied, the seismic refraction method interprets the arrival time of the refracted wave. By putting several hydrophones in a known configuration on the seabed, it is possible to calculate the velocity of an acoustic wave front through different layers of the subsoil. Sediment has sound velocity values, which typically range from ... 

The major advantage of the seismic refraction method is that the derived sound velocity of an individual layer correlates very well to the compactness of the sediment or to the Unconfined Compressive Strength rock strength (UCS). However, whereas the UCS value is a rock strength property the seismic sound velocity is more a rock mass characteristic that may strongly be affected by joints, fractures and other discontinuities. These properties in combination with the UCS determine the dredgeability of rock. 

A disadvantage of the seismic refraction method is that during processing of the data one has to assume that the rock layers show an increasing sound velocity with depth. Therefore, a hard caprock layer overlaying a softer rock cannot correctly be detected. 

About 70 % of the world is covered by oceans. Because of the difficulty of accessing the ocean floor, most of the seafloor and the crust below was unexplored for a long time. In the early 1930s, the seismic refraction method was developed and geoscientists tried to develop techniques to use this method offshore. They experimented with cabled sources and geophones but also with free-fall instruments. The first layout of a standalone ocean-bottom seismometer (OBS) was published in 1938 (Ewing and Vine 1938) and tested in the years 1939-1940. This OBS used a gasoline-filled rubber balloon for buoyancy, which floats approx. 3 m above the seafloor. 

Ivanova, M. A., A. D. Nikolov, I. B. Ivanov, L. Petrova, Refraction method for determining the degree of ethoxylation of nonionic surfactants, J. Dispersion Sci. TechnoL, 1995,16,495-510. 

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